Chromium-containing soils and tailings are significant environmental problems in many parts of the United States and around the world. Chromium compounds are used in metal cleaning, preplating and electroplating, as well as in the manufacture of inks, paint pigments and dyes. The chromium contamination is also a result of chromite ore processing, which generated large volumes of tailings and residues that have high concentrations of hexavalent chromium (Cr(VI)). These tailings and residues have been deposited as fill materials in many locations in and near the chromium manufacturing sites. As hexavalent chromium is a Class-A human carcinogen through inhalation, detoxification of these wastes will result in reduced human health risks and allow for future land use at or near known contaminated sites.
The United States Environmental Protection Agency (EPA) has established testing criteria for determining acceptable levels of chromium in the soil. In 1990, it was specifically established as part of the Resource Conservation and Recovery Act (RCRA) that the total chromium concentration in the leachate of the soil must fall below the standard Toxicity Characteristic Leaching Procedure (TCLP) limit of 5 mg/L in order for the soil to be no longer characterized as “hazardous waste.” Additionally, the groundwater chromium concentration level is regulated. Currently, the maximum contamination level (MCL) for total chromium is 100 ppb (parts per billion) or micrograms/liter. The California Department of Health Services (CDHS), however, disagrees with EPA on the toxicity of chromium and has set the State MCL to be 50 ppb and is proposing to regulate the groundwater hexavalent chromium at 0.2 ppb and total chromium at 2.5 ppb. These newer, more stringent levels are being considered to become effective in 2004 in California.
Brownfields, or pre-used urban industrial sites, are a type of location which may be contaminated with hexavalent chromium. If properly assessed or cleaned up, such sites could be reused for industrial purposes, which is desirable over utilizing new clean, suburban sites (greenfields) for similar purposes. Many brownfields are being cleaned up based on the calculated risks associated with various exposure scenarios. Typically, these are based on state laws or regulations rather than on federal laws or regulations. According to the most recent EPA Region III risk based criteria (RBC) guidelines, which were issued in April, 2000, the screening concentration for hexavalent chromium is 6,100 mg/Kg for industrial sites, 230 mg/Kg for residential areas, and 1.5×10−4 μg/m3 in ambient air. The cleanup targets for brownfields are often based on these less stringent cleanup standards based on hexavalent chromium concentration.
The most feasible method for the detoxification of hexavalent chromium is via the well-known reduction to trivalent chromium, Cr(III). Unlike Cr(VI), which is highly soluble, Cr(III) is not a human carcinogen and is typically found in insoluble forms in the environment. Cr(III) thus represents a lesser health concern than Cr(VI).
The reactions for the reduction of hexavalent chromium to trivalent chromium in aqueous solution are known. In addition, there are varying methods in the prior art to attempt to treat and stabilize chromium ore waste, which typically include the use of biological or chemical reduction. Bioremediation processes facilitate the reduction of Cr(VI) to Cr(III) through the use of anaerobic bacteria, whereas chemical reduction methods involve the addition of reducing agents and other reagents to the soil or material to be detoxified.
Many known processes designed for the reduction of Cr(VI) in soil and other waste materials are known as ex-situ methods, in which the soil must be excavated and fed through a reactor or apparatus for treatment. In a typical ex-situ process, such as that disclosed by U.S. Pat. No. 5,304,710, the soil, once excavated, is placed in a reactor and ground. The pH of the soil is then adjusted to an appropriate level and combined with a reduction agent, typically ferrous sulfate, to reduce the hexavalent chromium. Assuming that ferrous sulfate is used as the reducing agent, the following redox reaction applies:CrO42−+3Fe2++4H2O→Cr3++3Fe3++8OH−Following reduction, further treatments, such as neutralization, may be performed on the soil. The drawbacks generally of ex-situ processing methods are that large reactors must be constructed and the soil to be treated must be excavated and transported to the reactor for treatment, processes which are not efficient on a large scale and can be very costly and hazardous with respect to the transfer of contaminated materials.
In-situ methods of soil detoxification are more practical, cost effective and safer, especially when large areas of land must be treated. In this type of approach, one or more reagents are added to the soil (e.g., in the field) to bring about the reduction. Clear advantages are the elimination of both the reactor and the need for excavation. One such method is disclosed by U.S. Pat. No. 5,951,457 and involves the addition of ascorbic acid to the soil to reduce the Cr(VI) to Cr(III). In order to ensure that the chromium in the soil below the ground level is reduced, the soil must be mechanically mixed with the ascorbic acid. Although this method is designed to treat soil significantly below the ground level, extremely large quantities of the acid are necessary. As a result, the process is not economically feasible on a large scale due to the high costs of purchasing and transporting large quantities of ascorbic acid.
Other methods have been proposed for the addition of chemical reducing agents to the soil. These include (1) first drilling holes in the ground prior to introducing the reagent; and (2) utilizing a rototiller or similar device to thoroughly mix the soil with the reducing agent. One such method, directed toward the reduction of Cr(VI), is described in U.S. Pat. No. 5,285,000. However, delivery methods designed to inject solutions into soil are typically not effective methods of delivery because they do not typically provide even distribution of the reagent to the targeted contaminants. Additionally, the process involves dissolving and mixing ferrous and ferric salts in large quantities of water to produce the reducing solutions, which is likely to be quite costly.
A further such method in U.S. Pat. No. 5,397,478 is directed to the in-situ reduction of Cr(VI) in soil. This patent demonstrates the use of hole-drilling only on a very small test plot of soil in a laboratory. It does not provide guidance on how to feasibly implement such techniques practically on a large land area, in which the depth of the soil is significant, and/or in which large volumes of soil would be required to be mixed with or otherwise contacted with the reducing agents.
Bioremediation processes can also be performed in-situ. One such process, described in U.S. Pat. No. 5,681,639, involves stimulating the growth of indigenous anaerobic Cr(VI) reducing bacteria in the contaminated soil and/or groundwater by adding a nutrient medium to the soil and maintaining a substantially anaerobic environment. Such nutrients may be carbohydrates, amino acids, organic acids or nitrogen sources. However, no reliable means of controlling the biological reaction is described.
Even in view of the above described methods, there remains a need in the art for a method of in-situ soil remediation which is workable, safe, controllable, effective, and economically feasible and which can be applied on a large scale. There is a further need for an environmentally compatible process which is able to achieve soil chromium concentration levels below the TCLP limit and groundwater chromium concentration below regulatory levels, using inexpensive, easily available reagents and without the need for excavation or mixing of enormous volumes of soil with the applicable chemical reagents.